text: During the Three Mile Island accident, a slow partial meltdown of the reactor core occurred. About  of material melted and relocated in about 2 minutes, approximately 224 minutes after the reactor scram. A pool of corium formed at the bottom of the reactor vessel, but the reactor vessel was not breached. The layer of solidified corium ranged in thickness from 5 to 45 cm.
  Samples were obtained from the reactor. Two masses of corium were found, one within the fuel assembly, one on the lower head of the reactor vessel. The samples were generally dull grey, with some yellow areas.
  The mass was found to be homogeneous, primarily composed of molten fuel and cladding. The elemental constitution was about 70 wt.% uranium, 13.75 wt.% zirconium, 13 wt.% oxygen, with the balance being stainless steel and Inconel incorporated into the melt; the loose debris showed somewhat lower content of uranium (about 65 wt.%) and higher content of structural metals. The decay heat of corium at 224 minutes after scram was estimated to be 0.13 W/g, falling to 0.096 W/g at scram+600 minutes. Noble gases, caesium and iodine were absent, signifying their volatilization from the hot material. The samples were fully oxidized, signifying the presence of sufficient amounts of steam to oxidize all available zirconium.
  Some samples contained a small amount of metallic melt (less than 0.5%), composed of silver and indium (from the control rods). A secondary phase composed of chromium(III) oxide was found in one of the samples. Some metallic inclusions contained silver but not indium, suggesting a sufficiently high temperature to cause volatilization of both cadmium and indium. Almost all metallic components, with the exception of silver, were fully oxidized; even silver was oxidized in some regions. The inclusion of iron and chromium rich regions probably originate from a molten nozzle that did not have enough time to be distributed through the melt.
  The bulk density of the samples varied between 7.45 and 9.4 g/cm (the densities of UO and ZrO are 10.4 and 5.6 g/cm). The porosity of samples varied between 5.7% and 32%, averaging at 18±11%. Striated interconnected porosity was found in some samples, suggesting the corium was liquid for a sufficient time for formation of bubbles of steam or vaporized structural materials and their transport through the melt. A well-mixed (U,Zr)O solid solution indicates peak temperature of the melt between .
  The microstructure of the solidified material shows two phases: (U,Zr)O and (Zr,U)O. The zirconium-rich phase was found around the pores and on the grain boundaries and contains some iron and chromium in the form of oxides. This phase segregation suggests slow gradual cooling instead of fast quenching, estimated by the phase separation type to be between 3–72 hours.